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Did you accidently replace the tank with the rong gas? What are you running your gas at? What type and size of cup are you using? Did you check the hoses for leaks, holes, or cuts? What is the metal your welding on and is it clean or full of oil, rust, grease, grime, etc.....? Is the back cap and collet body on the torch tightened properly? All things to consider.

I do the same thing to let the pressure out of my machine at the end of the day...Turn off the gas, hit the pedal, etc. The hissing I hear is the gas flowing...you don't hear it when actually welding.

Anyway, there's a gas solenoid in there somewhere that may be the problem.

The fact that's it's not welding right means you probably have a gas flow problem. It might be that your close to being out of gas, the flowmeter isn't adjusted properly, the bottle is only barely open, aforementioned solenoid, etc.

Check, and double check the valves and settings, turn them up, down and back to normal, remove the torch and blow out the line (gently), verify gas is coming out when you hit the pedal, and go from there.

high flow rate = loud solenoid

whats the flow rate set at??? are you using a gas lens ?too high of a flow can cause turbulence and suck in oxygen that will cscrew up the weld fast.
500 psi is getting purity low for argon, what size tank are you using??
other possible causes of gas contamination.
is the valve on the tank is all the way open?? it must be tightly sealed all the way open, the argon tanks only seal if all the way open or all the way closed.
the back cap is lose on the torch.
the cup/seal are not seated properly.
the gas line's connections are not all tight.start off with a spray bottle of win-dex or orange clean . spray down all connections, even the top of the tank valve handel( i had a leak there once). if nothing bubbles its not a leak causing the problem in the weld.
also do not use PVC tape in any of the connections, they are presher fittings. tape is a no no!!

any of the above could cause the bad welds.

as for the noise ?? could just be normal solenoid flow noises, higher flow rates will make it louder and like said above you will generally not here it wile welding as you are buisy dealing with the weld.

good luck, be shore to let us know how it all comes out.

thanks for the help
......or..........
hope i helped feel free to shoot me an e-mail direct i have time to chat.james@newyorkmetalart.comsummer is here, plant a tree. if you don't have space or time to plant one sponsor some one else to plant one for you. a tree is an investment in our planet, help it out.
JAMES